Photonic crystals, also commonly referred to as photonic bandgap structures, are periodic dielectric or metallic structures exhibiting a spatially periodic variation in refractive index that forbids propagation of certain frequencies of incident electromagnetic radiation. The photonic band gap of a photonic crystal refers to the range of frequencies of electromagnetic radiation for which propagation through the structure is prevented. The photonic band gap phenomenon may be conceptualized as complete reflection of incident electromagnetic radiation having selected frequencies and propagating in certain directions due to interaction with the periodic structural domains of a photonic crystal. The spatial arrangement and refractive indices of these structural domains generate photonic bands gaps that inhibit propagation of electromagnetic radiation centered about a particular frequency.
Photonic crystals provide an electromagnetic analog to electron-wave behavior observed in crystals wherein electron-wave concepts, such as dispersion relations, Bloch wave functions, van Hove singularities and tunneling, having electromagnetic counterparts in photonic crystals. In semiconductor crystals, for example, an electronic band gap of energy states for which electrons are forbidden results from a periodic atomic crystalline structure. By analogy, in a photonic crystal, a photonic band gap of forbidden energies (or wavelengths/frequencies) of electromagnetic radiation results from a periodic structure of a dielectric material where the periodicity is of a distance suitable to interact with incident electromagnetic radiation of a selected wavelength.
Selection of the physical dimensions, refractive indices and spatial distribution of structural domains of a photonic crystal provides an effective means of designing a photonic crystal having a photonic band gap with a selected frequency distribution. One-dimensional, two-dimensional and three-dimensional photonic crystals have been fabricated providing complete or at least partial photonic bands having selected frequency distributions gaps in one or more directions. Photonic crystals have also been fabricated having selected local disruptions (e.g., missing or differently-shaped portions of the structural domains of periodic array) in their periodic structure, thereby generating defect or cavity modes with frequencies within a forbidden bandgap of the crystal. Photonic crystals having specific defects are of particular interest because they provide optical properties useful for controlling and manipulating electromagnetic radiation, such as the ability to provide optical confinement and/or wave guiding with very little, or essentially no, radiative losses.
As diffraction and optical interference processes give rise to the photonic band gap phenomenon, the periodicity of photonic crystal structures is typically on the order of the wavelength of incident electromagnetic radiation. Accordingly, photonic crystals for controlling and manipulating visible and ultraviolet electromagnetic radiation typically comprise dielectric or metallic structures with periodic structural domains having submicron physical dimensions on the order of 100 s nanometers. A number of fabrication pathways for making periodic structures having these physical dimensions have been developed over the last decade, including micromachining and nanomachining techniques (e.g., lithographic patterning and dry/wet etching, electrochemical processing etc.), colloidal self assembly, layer-by-layer assembly and interference lithography. Advances in these fabrication techniques have enabled fabrication of one-dimensional, two-dimensional and three-dimensional photonic crystals from a range of materials including dielectric crystals, metallic, polymeric and colloidal materials.
The structure, composition, fabrication and optical properties of photonic crystals are described in the following references which are hereby incorporate by reference in their entireties: (1) Joanopoulus et al., “Photonic Crystals Molding the Flow of Light”, Princeton University Press, 1995; (2) A. Birner, R. B. Wehrspohn, U. M. Gösele, K. Busch, “Silicon-Based Photonic Crystals”, Advanced Materials, Volume 13, Issue 6, Pages 377-388; and (3) Steven G. Johnson, and John D. Joannopoulos, “Photonic Crystals: The Road from Theory to Practice”, Springer, 2002.
Given recent advances in their fabrication and their unique optical properties, photonic crystals are identified as key components for realizing a new generation of high performance, low loss optical and electro-optic devices. As an alternative to semiconductor technologies, photonic crystals have great potential to provide a promising pathway to a range of smaller, faster and more energy efficient devices that perform the same functionality as their silicon-based counterparts. Accordingly, photonic crystals have the potential to revolutionize a number of technologies ranging from optical computing, dense wavelength division multiplexing, light emitting systems and biosensing. A number of passive photonic devices have been fabricated taking advantage of the complete and partial photonic band gap(s) provided by photonic crystals, including optical filters, beam splitters, waveguides, channel drop filters and resonance cavities.
Photonic crystals also have great potential as components in active photonic devices, such as solid state lasers, optical switches, optical diodes and optical transistors. To expand their functionality, role and applicability in active photonic device configurations, substantial research is currently being directed at developing photonic crystal structures and systems providing a selectively tunable photonic band gap. Tunability in this context refers to the ability to selectively change the range of frequencies corresponding to a photonic band gap of a photonic crystal. The ability to dynamically control (i.e., selectively tune) the frequency range of a photonic band gap on a fast time scale (e.g. milliseconds or less) would potentially enable optical switches and transistors for a range of important applications including optical signal processing in telecommunications, all-optical integrated circuits, all-optical computing applications and information storage. A number of approaches for providing photonic crystals with tunable photonic band gaps have been pursued including: (i) incorporation of nematic liquid crystal materials and/or conducting organic polymers responsive to applied electric fields into the periodic dielectric structures of photon crystals; (ii) colloidal photonic crystals comprising thermo- or electro-responsive hydrogel nanoparticles; (iii) use of flexible, expandable and/or compressible photonic crystals capable of changing periodicity upon application of mechanical stress; and (iv) coupled photonic crystal systems having a mechanically tunable air separation layer. While these approaches have met with some degree of success, tunable photonic crystals currently do not exhibit the high level of performance (e.g., fast modulation rate, high optical throughput and low loss) required for many applications and typically involve expensive fabrication pathways that are not generally amenable to low cost, mass production and commercialization.
Given their great potential for active and passive components in a range of useful devices, it will be appreciated that there is currently a need for new photonic crystal based devices, systems and instrumentation. It will also be appreciated from the foregoing that a need exists for high performance tunable photonic crystals capable of fast optical modulation and compatible with commercially practicable fabrication methods.